Method of forming sheet or plate material



Sept. 22, 1970 B L ET AL 3,529,458

METHOD OF FORMING SHEET on PLATE MATERIAL Filed Dec. 15, 1967 3,529,458 METHOD OF FORMING SHEET OR PLATE MATERIAL Roger David Butler, Kidlington, Ian Frederick Bowers,

Freeland, and Cedric Charles Edward Colley, Wheatley,

England, assignors to Pressed Steel Fisher Limited,

Cowley, Oxford, England, a British company Filed Dec. 15, 1967, Ser. No. 691,004 Int. Cl. B21d 26/04 U.S. Cl. 7260 3 Claims ABSTRACT OF THE DISCLOSURE After forming superplastic sheet or plate material by heating the material to within the superplastic temperature range and then forming the material using at least one die, cooling gas is blown through the die to stiffen the formed material and then the gas pressure is increased to strip the formed material from the die; advantageously gas is also passed through the die during forming to produce a gas bearing or cushion on the forming surface of the die.

This invention relates to a method of forming, or shaping, sheet or plate material that exhibits superplasticity within a limited range of temperatures.

It is knovm that within a limited range of temperatures certain metal alloys, such as the eutectoid alloy of aluminium and zinc, and the eutectic alloy of aluminium and silicon, may be in a state such that they have especially low values of resistance to deformation and extremely high plasticity as compared with other similar alloys having different proportions of the elements, or to the same alloy in a different state or at a temperature outside said range. This phenomenon occurs below a certain strain rate at any particular temperature within the said temperature range and is known as superplasticity, and for convenience metal alloys which are capable of exhibiting this phenomenon are termed superplastic in this specification.

The object of this invention is to provide a method of forming or shaping superplastic sheet or plate material.

According to the invention a method of forming superplastic sheet or plate material by heating the material to within the superplastic temperature range and then forming the material using at least one die, includes so passing a gas through the die that the formed material is cooled, and then so increasing the gas pressure that the formed material is stripped from the die surface; the material being cooled to a temperature at which it is sufficiently rigid not to be permanently deformed during stripping.

Preferably the method also includes so passing gas through the die during forming that the gas generates a gas bearing or cushion on the surface of the die.

The gas may be passed through ducts formed in the die or the gas may be passed through a porous die.

An exemplary embodiment of the invention will now be described, solely by way of illustration with reference to the accompanying drawing which is a schematic, partially-sectioned, elevation of apparatus for effecting the invention.

The forming apparatus shown in the drawing includes a restraining frame 11 comprising a top platen 12 connected by tie posts, such as 13, to a lower platen 14.

A two-way, pneumatic piston and cylinder device 15 having an inlet 16 and an outlet 17, supports a mould casing 18.

Previously cast in the mould casing 18 is a concrete die 19 having a forming surface 20 and incorporating a compressed air inlet duct 21 and electric resistance heating coils 22 the duct 21 being connected to a controlled source of compressed air (not shown) and the coils 22 being con- I United States Patent 3,529,458 Patented Sept. 22, 1970 trolled by a thermocouple temperature controlling device (not shown) responsive to temperature adjacent the forming surface 20. The die 19 is fitted with a steel sealing flange 23 having deep narrow radial slots (not shown) formed on the outer, workpiece-holding, face.

The lower platen 14 supports a concrete back-up block 24 in which is embedded a steel forming chamber 25. Electric resistance heating coils 26 are embedded in the concrete block 24 adjacent the chamber 25 and are controlled by a thermocouple enclosed in a metal or refractory sheath 27 extending into the chamber 25 and connected to a temperature controlling device (not shown).

A high frequency signal generator 28 has a horn 29 which extends into the chamber 25, and is sealed by a high temperature O-ring 30.

The chamber 25 is connected by a pipe 31 to a reservoir 32. The reservoir 32 is surrounded by electric resistance heating coils 33 controlled by a thermocouple enclosed in a metal or refractory sheath 34 and connected to a temperature controlling device (not shown), and the pipe 31 is surrounded by further electric resistance heating coils 35 of which those adjacent the reservoir 32 are controlled by a thermocouple in the sheath 34 whilst those adjacent the forming chamber 25 are controlled by the thermocouple in the sheath 27.

The top of the reservoir 32 is connected by the pipe 36 and through a pressure regulating valve 37 to a compressed air source inlet 39, and is also connected by the branch pipe 38 through a metering valve 40 to a variable preset metering pump 41. Furthermore the pipe 36 is provided with a pressure release valve 42.

The reservoir 32, the pipe 31 and the forming chamber 25 are filled with a low melting fusible metal alloy 43 having a melting point of C. and maintained at 260 C. by the resistance heaters 26, 33 and 35; and the forming surface 20 of the die 19 is also maintained at 260 C. by the heating coils 22.

In operation, a superplastic sheet workpiece 44 is placed as shown in the drawings; the workpiece may be formed of the eutectoid alloy of zinc and aluminium which has been prepared by quenching the sheet in water from 325-350 C. to room temperature in order to induce the required superplastic state.

In this example the workpiece 44 has been pre-stretched by conventionally forming it in a press to the shape shown in the drawing. The workpiece 44 is placed in position as shown and the pneumatic device 15 is operated to move the die 19 towards the forming chamber 25 until the sealing flange 23 is loosely holding the workpiece 44; the release valve 42 is closed, and the preset metering pump 41 is actuated to increase the pressure on the hot alloy 43 sufficiently to cause the alloy 43 in the forming chamber 25 to contact the workpiece 44 and thereby raise the temperature of the workpiece to 260 C., the air within the chamber 25 being displaced past the periphcry of the workpiece 44.

The device 15 is then operated to firmly grip the workpiece 44 and effect a seal between the workpiece 44 and the fiuid chamber 25. The signal generator 28 is energised and high pressure compressed air is admitted through the inlet 39, the pressure regulating valve 37 and the pipe 36 into the reservoir 32, and thereby the hot alloy 43 in the forming chamber 25 forces the workpiece 44 towards the surface 20 of the die.

Compressed air is then admitted through the duct 21 to generate an air bearing or cushion on the surface 20 of the die 19. It will, of course, be understood that the ducting arrangement shown in the drawings is purely schematic, since the detailed design will depend on the various parameters individual to each workpiece. How ever as a generality the arrangement which will normal- 1y include a plurality of ducts connected to associated channels, is such that as a specific region of the workpiece is forced into close proximity with the die surface, the pressure of compressed air acting on that region is slightly in excess of the pressure of the hot alloy 43 by an amount which is sufficient to prevent contact of the workpiece with the die, but is insufficient to deform the shaped region of the workpiece; over the remainder of the workpiece the pressure of the compressed air will be lower than the forming pressure of the hot alloy 43 due to the relatively free escape of compressed air to the slots in the flange 23 which act as exhaust slots and which have a total exhaust area larger than the combined area of the compressed air inlets. Thus a pressure gradient exists from the region of the workpiece that is in close proximity to the die towards the exhaust slots, and hence final forming takes place progressively from the said regions to the exhaust slots.

During forming it will be understood that the air pressure in the pipe 36 and restrictions to flow must be such that the critical value of strain rate is not exceeded.

After the workpiece 44 has been formed to the required shape, the electric heating coils 22 are turned oflf, and the pressure release valve 42 is opened to allow the level of the hot alloy 43 in the forming chamber 25 to fall and simultaneously the pressure applied to the duct 21 is also allowed to fall.

The pressure in the pneumatic device 15 is then re leased.

Compressed air is then admitted at low pressure through the duct 21 to cool the workpiece 44 until it is sufficiently rigid to resist permanent deformation on stripping from the die 19.

Stripping is then effected by increasing the pressure until the workpiece is stripped from the surface of the die.

The pneumatic device 15 is then operated to lift the die 19, and the workpiece is removed.

What we claim is:

1. The method of forming superplastic sheet or plate workpiece material which includes the following steps:

(a) heating the workpiece to within its superplastic temperature range,

(b) deforming the workpiece at superplastic strain rates by forcing it against a die,

(0) removing the deforming force,

(d) expelling gas from the surface of the die so that the gas flows across substantially the entire surface of at least one face of the formed workpiece and towards the periphery thereof, thereby cooling the sheet,

(e) then increasing the gas pressure that the workpiece is stripped from the die.

2. The method of claim 1 including the additional step (a) expelling gas from the surface of the die recited at step (b) at such a rate that a cushion of air is maintained between the workpiece and the die during the deformation of the workpiece but at a rate insuflicient to deform the die shaped regions of the workpiece.

3. The method of claim 2 wherein the step (a) takes place with a radially outwardly diminishing pressure gradient between the die and the workpiece.

References Cited UNITED STATES PATENTS 1,737,874 12/1929 Busch 18--19 3,172,928 3/1965 Johnson 7256 3,340,101 9/1967 Fields et al. 72364 OTHER REFERENCES Superplasticity in an Al-Zn Alloy by W. A. Backofen et al., pp. 980-990, vol. 57, 1964, Transactions of the ASM.

RICHARD J. HERBST, Primary Examiner U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,529,458 Dated ep e 22, 1970 Inventor) R. D. Butler et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claims priority, application Great Britain,

December 23, 1966 57,755/66; March 15, 1967 12068/67.

SEAL) Amen M will-Ill. I manner 01 Pat-ants ILS, GOVEINIINY PRINTING DFI'ICE: III! 0-306-881 

